Move wxQT notebook helper from header to source (make it internal)
Fix notebook tab change events and insertion (move selection logic from qt helper to wx class, and block firing qt signals to avoid initialization problems and to better simulate signal vetoing -to not show the page in advance-). Solution for selection logic taken from wxGTK (now using base clases). Note also that most methods of QTabWidget cannot be overriden (IMHO enforcing the thin qt helpers approach).